A new method is presented to predict the compatibility of a polymer blend. The compatibility of the polymer-polymer pairs containing poly(vinyl chloride) (PVC), poly(glycidyl methacrylate) [poly(GMA)], poly(methyl methacrylate) [poly(MMA)], and poly(styrene) (PS) predicted by the use of the deviation from the additivity law of the slope in the reduced viscosity (eta(sp)/C) versus polymer concentration (C) plot of a binary blend system is the purpose of this article. In addition, the influence of the grafted copolymer (PVC-g-GMA) on the compatibility of PVC-poly(GMA) pairs is also investigated by this reduced viscosity method and also confirmed by the differential scanning calorimetric technique. The results contrasted to the thermodynamic principle showed that the polymer blends could be considered to be a compatible blend when the deviation values deviated from the additivity law of the slopes in the eta(sp)/C vs. C plot are smaller than 15%.